Automated machines, for the insertion of leaded components into thru-hole locations on printed circuit boards (PCB), have been in use for more than twenty years. In spite of the maturity of this type of equipment, the insertion process continues to be subject to a relatively high misinsertion rate. This observation is further emphasized by the fact that most machines are equipped to sense when component leads are not successfully inserted through the appropriate holes in the printed circuit boards and to stop the machine when component lead misinsertion occurs. Unfortunately, this fault protection procedure requires operator intervention to restart the machine, causes degradation in productivity of the process, and causes scrapping of a component on each retry. Often, the cause of the misinsertion is fundamental, requiring subsequent hand insertion of the component and associated potential for defects through the waveline or beyond. There are several fundamental reasons for this high misinsertion rate.
First, the insertion process itself is a relatively complex process subject to numerous sources of variation. These include the machine itself, the PCB, and the components. Within the PCB, the tooling holes used to register the board and the component holes designed to accommodate the component leads can and do vary independently. Within the insertion machine, both the X-Y table and the insertion head contribute independently to system variation. Finally component variation can occur in the accuracy of the spacing of the parts on the tape and reel, component body dimensions, lead dimensions, and lead composition or stiffness. The nature of all of these sources of variation is multifaceted and interdependent, making the activity of process optimization and problem solving extremely difficult. This difficulty is compounded by the lack of a method which can parametrically measure the accuracy of the total machine/component system separately from the PCB part of the system.
Secondly, thru-hole insertion is intrinsically a more difficult process than, for instance, surface mounted component placement. To illustrate very simplistically, leaded parts require the insertion of a 0.024" diameter lead into a 0.042" diameter hole, giving a tolerance window of .+-.0.009". For chip placement, the same tolerance window is .+-.0.015" for SOT-23's and .+-.0.020" for chips. Thus the dimensional accuracy required to achieve the same quality level is significantly tighter for component insertion than it is for chip placement.
Lastly, the consequences of missing the target for insertion are different. A misinsertion stops the insertion machine regardless of the magnitude of the miss. For placement, a part marginally out of the tolerance window will usually result in acceptable soldering and will not cause the machine to stop.
Chip placement equipment has been availabe for about ten years. In spite of this significantly lower maturity, chip machines are more accurate than their insertion counterparts. One of the reasons that this is the case is that chip placement accuracy can be measured parametrically, allowing quick comparison of machine design alternatives. Until now, there has been no analogous capability for assessing insertion machine accuracy. Accordingly, a need exists to characterize parametrically, under use conditions, insertion machine accuracy.